Abstract
The diffusion of molecules in interstellar ice is a fundamental phenomenon to take into account while studyingthe formation of complex molecules in this ice. This work presents a theoretical study on the diffusion of H2O,NH3, CO2, CO, and H2CO in the bulk of a low density amorphous (LDA) ice, while taking into account thephysical conditions prevailing in space, i.e. temperatures below 150 K and extremely low pressure. This studywas undertaken by means of molecular dynamics simulations. For CO2 for which no experimental data wereavailable we conducted our own experiments. From our calculations we show that, at low temperatures, thediffusion of molecules in the bulk of a LDA ice is driven by the self-diffusion of water molecules in the ice. Withthis study we demonstrate that molecular dynamics allows the calculation of diffusion coefficients for smallmolecules in LDA ice that are convincingly comparable to experimentally measured diffusion coefficients. Wealso provide diffusion coefficients for a series of molecules of astrochemical interest.